U.S. patent number 9,080,463 [Application Number 13/255,436] was granted by the patent office on 2015-07-14 for turbine ring assembly.
This patent grant is currently assigned to HERAKLES, SNECMA. The grantee listed for this patent is Damien Bonneau, Franck Roger Denis Denece, Alain Dominique Gendraud, Georges Habarou, Hubert Illand. Invention is credited to Damien Bonneau, Franck Roger Denis Denece, Alain Dominique Gendraud, Georges Habarou, Hubert Illand.
United States Patent |
9,080,463 |
Denece , et al. |
July 14, 2015 |
Turbine ring assembly
Abstract
A turbine ring assembly includes a ring support structure and a
plurality of ring sectors, each including a single piece of ceramic
matrix composite material. Each ring sector includes a first
portion forming an annular base with an inside face defining an
inside face of the turbine ring and an outside face from which
there extends two tab-forming portions including ends that are
engaged in housings in the ring support structure. The ring sectors
present a section that is substantially .pi.-shaped and the ends of
the tabs are held without radial clearance by the ring support
structure. The tabs can have a free length in meridian section that
is not less than three times their mean width.
Inventors: |
Denece; Franck Roger Denis
(Saint Michel sur Orge, FR), Gendraud; Alain
Dominique (Vernou la Celle sur Seine, FR), Habarou;
Georges (Le Bouscat, FR), Illand; Hubert
(Bonchamp, FR), Bonneau; Damien (Melun,
FR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Denece; Franck Roger Denis
Gendraud; Alain Dominique
Habarou; Georges
Illand; Hubert
Bonneau; Damien |
Saint Michel sur Orge
Vernou la Celle sur Seine
Le Bouscat
Bonchamp
Melun |
N/A
N/A
N/A
N/A
N/A |
FR
FR
FR
FR
FR |
|
|
Assignee: |
SNECMA (Paris, FR)
HERAKLES (Le Haillan, FR)
|
Family
ID: |
42237201 |
Appl.
No.: |
13/255,436 |
Filed: |
March 1, 2010 |
PCT
Filed: |
March 01, 2010 |
PCT No.: |
PCT/FR2010/050342 |
371(c)(1),(2),(4) Date: |
October 20, 2011 |
PCT
Pub. No.: |
WO2010/103213 |
PCT
Pub. Date: |
September 16, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120027572 A1 |
Feb 2, 2012 |
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Foreign Application Priority Data
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Mar 9, 2009 [FR] |
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09 51445 |
Mar 9, 2009 [FR] |
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09 51446 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01D
11/08 (20130101); D03D 25/005 (20130101); F01D
9/04 (20130101); F01D 25/246 (20130101); Y02T
50/60 (20130101); Y02T 50/67 (20130101); F05D
2240/11 (20130101); F05D 2300/21 (20130101); F05D
2260/30 (20130101); F05D 2230/642 (20130101); F05D
2300/603 (20130101); Y02T 50/672 (20130101) |
Current International
Class: |
F01D
25/24 (20060101); F01D 9/04 (20060101); F01D
11/08 (20060101) |
Field of
Search: |
;415/119,139,168.4,173.1,173.6,173.4,174.4,200 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 099 826 |
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May 2001 |
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EP |
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1 225 309 |
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Jul 2002 |
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EP |
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2 919 345 |
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Jan 2009 |
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FR |
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2 445 075 |
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Jun 2008 |
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GB |
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Other References
International Search Report issued Jun. 29, 2010 in PCT/FR10/50342
filed Mar. 1, 2010. cited by applicant.
|
Primary Examiner: Look; Edward
Assistant Examiner: Christensen; Danielle M
Attorney, Agent or Firm: Oblon, McClelland. Maier &
Neustadt, L.L.P.
Claims
The invention claimed is:
1. A turbine ring assembly comprising: a ring support structure and
a plurality of ring sectors, each comprising a single piece of
ceramic matrix composite material, each ring sector including a
first portion forming an annular base with an inside face defining
an inside face of the turbine ring and an outside face from which
there extends upstream and downstream tab-forming portions
including upstream and downstream ends, respectively, that are
engaged in housings in the ring support structure, wherein the ring
sectors present a section that is substantially .pi.-shaped and the
upstream and downstream ends of the tab-forming portions are held
without radial clearance by the ring support structure, wherein an
upstream end of the ring support structure includes a radial flange
including a hook of annular section with a U-shaped section that is
open in a downstream axial direction, the hook presenting opposite
inner and outer annular branches, the outer branch being longer
than the inner branch, wherein the upstream end of the upstream tab
is engaged between an inner face of the inner branch and an inner
face of the outer branch, wherein the inner face of the outer
branch presents a setback such that a radial distance between the
inner faces of the branches in a vicinity of an opening of the hook
is smaller than a radial distance between the inner faces of the
branches in a vicinity of a bottom of the hook, wherein a radial
thickness of the upstream end of the upstream tab is substantially
equal to the radial distance between the inner faces of the
branches in the vicinity of the opening of the hook, wherein the
tab-forming portions have a free length in meridian section that is
not less than three times their mean width, wherein each ring
sector is held axially by mutual engagement of substantially
complementary axial holding portions in relief formed on facing
bearing surfaces of a tab-forming portion and of a portion of the
ring support structure, and wherein the axial holding portion in
relief on the bearing surface of an attachment tab is in a form of
a groove co-operating with a rib formed on the bearing surface of
the ring support structure.
2. A turbine ring assembly according to claim 1, wherein the
tab-forming portions are substantially S-shaped in meridian
section.
3. A turbine ring assembly according to claim 1, wherein the
downstream end of the downstream tab-forming portion is held
radially without clearance against an annular surface of the ring
support structure by a fitted clip.
4. A turbine ring assembly according to claim 1, further comprising
a sealing gasket interposed between the facing bearing
surfaces.
5. A turbine ring assembly according to claim 1, wherein the groove
has a profile that is substantially V-shaped.
6. A turbine ring assembly according to claim 1, wherein a
downstream end of the inner face of inner branch presents a
chamfer.
7. A turbine ring assembly according to claim 1, wherein the inner
face of the inner branch has a rectilinear profile.
Description
BACKGROUND OF THE INVENTION
The invention relates to a turbine ring assembly for a
turbomachine, which assembly comprises a ring support structure and
a plurality of ring sectors, each comprising a single piece of
ceramic matrix composite material.
The field of application of the invention is particularly that of
gas turbine aeroengines. Nevertheless, the invention is applicable
to other turbomachines, e.g. industrial turbines.
Ceramic matrix composite (CMC) materials are known for their good
mechanical properties, which make them suitable for constituting
structural elements, and for their capacity to conserve those
properties at high temperatures.
In gas turbine aeroengines, improving efficiency and reducing
polluting emissions are leading pursuit of ever-higher operating
temperatures.
Thus, the use of CMCs for various hot portions of such engines has
already been envisaged, particularly since CMCs present density
that is less than that of the refractory metals that are
conventionally used.
Thus, making one-piece CMC turbine ring sectors is already
described in document U.S. Pat. No. 6,932,566. The ring sectors
have a K-shaped meridian section with an annular base in which the
inside face defines the inside face of the turbine ring and an
outside face from which there extend two tab-forming portions with
ends that are engaged in U-shaped housings in a metal structure for
supporting the ring. The tabs have their ends engaged with radial
clearance in the U-shaped housings and they are held to bear
radially against surfaces of the housings by means of a resilient
member that exerts a return force on the ring sector, which force
is directed radially towards the axis of the ring.
Engaging the ends of the tabs in the housings with clearance makes
it possible to accommodate differential expansion between the CMC
and the metal of the ring support structure, however the resilient
mounting presents several drawbacks.
Thus, such resilient mounting is poorly compatible with the
finishing machining that is conventionally performed after initial
mounting of the ring sectors in order to confer an almost perfect
cylindrical shape to the inside surface of the ring.
In addition, when the tip of a blade on a rotor wheel surrounded by
the ring comes into contact with an abradable coating present on
its inside face, the resilient mounting gives rise to an
undesirable vibratory phenomenon.
Furthermore, the sealing of the gas flow passage on the inside of
the ring sectors relative to the outside of the ring sectors is
affected.
OBJECT AND SUMMARY OF THE INVENTION
The invention seeks to avoid such drawbacks, and for this purpose
it proposes a turbine ring assembly comprising a ring support
structure and a plurality of ring sectors, each comprising a single
piece of ceramic matrix composite material, each ring sector having
a first portion forming an annular base with an inside face
defining the inside face of the turbine ring and an outside face
from which there extend two tab-forming portions having ends that
are engaged in housings in the ring support structure, in which
turbine ring assembly the ring sectors present a section that is
substantially .pi.-shaped and the ends of the tabs are held without
radial clearance by the ring support structure.
Thus, the turbine ring assembly is remarkable in that the CMC ring
sectors are held without radial clearance by the ring support
structure, and they present a shape such that they are held at a
location that is relatively far from the zone that is hottest in
operation.
Advantageously, the tabs have a free length in meridian section
that is not less than three times their mean width.
Also advantageously, the tabs are substantially S-shaped in
meridian section.
According to a feature of the turbine ring assembly, one, or a
first one, of the tabs has its end held radially by engaging in a
housing of substantially U-shaped section in a one-piece
hook-shaped portion of the ring support structure. By way of
example, this first tab is the upstream tab. Preferably, the
housing of U-shaped section is defined by opposite inner and outer
branches, and the inner branch has a length that is shorter than
the length of the outer branch.
According to another feature of the turbine ring assembly, one, or
a second one, of the tabs has its end held radially without
clearance against an annular surface of the ring support structure
by means of a fitted clip. By way of example, this second tab is
the downstream tab.
According to yet another feature of the turbine ring assembly, each
ring sector is held axially by mutual engagement of substantially
complementary axial holding portions in relief formed on facing
bearing surfaces of a tab and of a portion of the ring support
structure. A sealing gasket may be interposed between the facing
bearing surfaces. The axial holding portion in relief on the
bearing surface of an attachment tab may be in the form of a groove
co-operating with a rib formed on the bearing surface of the ring
support structure, the groove advantageously having a profile that
is substantially V-shaped.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be better understood on reading the following
description given by way of nonlimiting indication with reference
to the accompanying drawings, in which:
FIG. 1 is a meridian half-section view showing an embodiment of a
turbine ring assembly of the invention;
FIGS. 2 to 4 are diagrams showing the assembly of a ring sector in
the ring support structure of the FIG. 1 ring assembly;
FIGS. 5A and 5B are two three-dimensional weaving planes showing an
embodiment of a fiber blank for a CMC ring sector of the FIG. 1
ring assembly;
FIG. 6 shows a fiber preform for a CMC ring sector of the turbine
ring assembly of FIG. 1;
FIG. 7 shows successive steps in an implementation of a method of
making a CMC turbine ring sector; and
FIG. 8 is a meridian section view showing a variant embodiment of a
CMC ring sector for a turbine ring assembly of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
FIG. 1 shows a high-pressure turbine ring assembly comprising a CMC
turbine ring 1 and a metal ring support structure 3. The turbine
ring 1 surrounds a set of rotary blades 5. The turbine ring 1 is
made up of a plurality of ring sectors 10, FIG. 1 being a meridian
section view on a plane passing between two contiguous rings.
Each ring sector 10 has a section that is substantially .pi.-shaped
with an annular base 12 having an inside face coated in a layer 13
of abradable material defining the flow passage for the gas stream
through the turbine. Tabs 14, 16 having a substantially S-shaped
meridian section extend from the outside face of the annular base
12 over its entire length. One of the tabs, or upstream tab 14,
extends upstream, and its upstream end portion 14a is situated
upstream from the upstream end of the annular base 12. The other
tab 16, or downstream tab, extends downstream and its downstream
end portion 16a is situated downstream from the downstream end of
the annular base 12. The terms "upstream" and "downstream" are used
herein with reference to the flow direction of the gas stream
through the turbine (arrow F).
The ring support structure 3 that is secured to the turbine casing
30 comprises an annular upstream radial flange 32 carrying a hook
34 of annular section with a U-shaped meridian section that is open
in the downstream axial direction. The hook 34 presents opposite
inner and outer annular branches 34a, 34b. The annular upstream end
portion 14a of the tab 14 is engaged between the inner faces 35a,
35b of the branches 34a, 34b. The branch 34a of the hook 34
carrying the face 35a is shorter than the branch 34b carrying the
face 35b, the branch 35a thus terminating upstream from the end of
the branch 35b. The face 35a has a rectilinear profile, while the
face 35b presents a setback 35c, such that the radial distance d
between the faces 35a and 35b in the vicinity of the opening of the
hook is slightly smaller than the radial distance between the faces
35a and 35b in the vicinity of the bottom of the hook 34. The
distance d is equal to or very slightly smaller than the thickness
e of the end portion 14a of the tab 14, such that the end portion
14a of the tab 14 is engaged without clearance or even under a
certain amount of stress between the surfaces 34a and 34b in the
vicinity of the opening of the hook 34. In contrast, a small amount
of clearance j is left between the end portion 14a and the surface
34d in the vicinity of the bottom of the hook 34. It should also be
observed that a chamfer is formed at the downstream end of the face
35a.
Mounting the end portion 14a of the tab 14 in the hook 34 thus
serves to provide sealing between the flow passage for the gas
stream and the outside of the ring sectors, at the upstream ends
thereof.
At the downstream end, the ring support structure includes an
L-section annular flange 36 terminating in an annular bearing
portion 36a against which the annular end portion 16a of the tab 16
bears. The end portion 16a of the tab 16 and the bearing portion
36a of the flange 36 are kept pressed against each other without
clearance by means of a clamp 38 having a U-shaped meridian
section, in a manner that is itself known. The clamp is prevented
from moving circumferentially relative to the flange 36 and to the
tab 16 by being inserted between fingers 36b, 16b projecting
downstream from the portion 36a of the flange 36 and from the end
portion 16a of the tab 16.
The bearing portion 36a of the flange 36 presents a circumferential
rib 37 that projects inwards and that is received in a groove 17
formed in the outer annular face of the end portion 16a of the tab
16. The groove 17 has a section that is preferably substantially
V-shaped, while the rib has a section that is substantially
U-shaped or V-shaped. This serves to prevent the ring sectors from
moving in the axial direction relative to the ring support
structure.
In order to ensure the best possible sealing between the flow
passage for the gas stream through the turbine and the outside of
the turbine ring at the downstream end thereof, a gasket 20 is
compressed between the bearing portion 36a of the flange 36 and the
end portion 16a of the tab 16. By way of example, the gasket 20 is
constituted by a metal braid held in a housing formed in the inside
face of the bearing portion 36a downstream from the rib 37.
In addition, inter-sector sealing is provided by sealing tongues
housed in grooves that face each other in the facing edges of two
adjacent ring sectors. A tongue 22a extends over nearly the entire
length of the annular base 12 in its middle portion. Another tongue
22b extends along the tab 14. At one end, the tongue 22b comes into
abutment against the tongue 22a, while at the other end, the tongue
22b comes up to the top face of the end portion 14a of the tab 14,
preferably at a location where the tab is engaged without clearance
in the hook 34. Another tongue 22c extends along the tab 16. At one
end, the tongue 22c comes into abutment against the tongue 22a,
while at the other end, the tongue 22c comes up to the top face of
the end portion 16a of the tab 16, preferably at the location of
the gasket 20. By way of example, the tongues 22a, 22b, and 22c are
made of metal and they are mounted in their housings with clearance
when cold so as to provide the sealing function at the temperatures
they encounter in operation.
Assembling the tabs 14 and 16 of the CMC ring sector with the metal
portions of the ring support structure without relative clearance
is possible, in spite of the different coefficients of thermal
expansion, because: assembling is performed at a distance from the
hot face of the annular base 12 that is exposed to the gas stream;
and in their meridian sections, the tabs 14 and 16 advantageously
present a length that is relatively long compared with their mean
width, such that effective thermal decoupling is obtained between
the annular base 12 and the ends of the tabs 14 and 16,
particularly since CMC presents low thermal conductivity.
Furthermore, and in conventional manner, ventilation orifices 32a
formed through the flange 32 serve to bring cooling air in from the
outside of the turbine ring 1.
Preferably, the free length of the tabs is equal to at least three
times their mean width. The term "free length" is used herein to
mean the length of the profile in meridian section between the
connection with the annular base 12 and the contact with the
support structure.
FIGS. 2 to 4 show successive steps in assembling a ring sector. The
difference in axial length between the branches 34a, 34b, the
presence of the clearance j at the bottom of the hook 34, and the
presence of a chamfer at the end of the face 35a make it easier to
tilt the ring sector in order to pass the rib 37 (FIG. 2), a small
tilt angle of a few degrees sufficing. This avoids excessive
bending stress on the CMC ring sector.
When the groove 17 is facing the rib 37, the ring sector can be put
back into position (FIG. 3).
When the end portion 14a of the tab 14 is brought against the
bearing portion 36a of the flange 36 (FIG. 4), the end portion 14a
the tab 14 is pressed firmly against the face 35b of the branch 34b
of the hook 34 in the vicinity of its opening, bearing against the
opposite face 35a. The end portion 14a of the tab 14 is thus in
close contact with the faces 35a and 35b.
All of the sealing tongues 22a, 22b, and 22c may be put into place
before bringing all of the sectors 10 into the turbine casing. In a
variant, the sectors 10 may be mounted in the casing one by one
without tongues, and they may be successively spaced apart
circumferentially in order to insert the tongues.
Each ring sector 10 is made of CMC by forming a fiber preform of a
shape that is close to the shape of the ring sector and by
densifying the ring sector with a ceramic matrix.
In order to make the fiber preform, it is possible to use yarns of
ceramic fibers, for example yarns of SiC fibers such as those sold
by the Japanese supplier Nippon Carbon under the name "Nicalon", or
yarns of carbon fibers.
The fiber preform is advantageously made by a three-dimensional
weaving, or by multilayer weaving with non-interlinked zones being
left to make it possible for the portions of the preform that
corresponds to the tabs 14 and 16 to be spaced apart from the
portion of the preform that corresponds to the base 12.
FIGS. 5A and 5B in warp section show examples of successive weaving
planes for weaving a blank 100 suitable for obtaining a ring sector
preform.
In the example shown, the total number of layers of warp yarns is
equal to four. It could naturally be other than four, in particular
it could be greater. In a first plane (FIG. 5A), the warp yarn
layers are all interlinked by a weft yarn in the central portion of
the blank corresponding to the central portion of the ring between
its upstream and downstream ends, while each side of the central
portion has only the top two layers of warp yarns being
interlinked. In the following plane (FIG. 5B), the warp yarn layers
are still all interlinked by weft yarn in the central portion of
the blank, while on each side of the central portion only the two
bottom layers of warp yarns are interlinked.
It should be observed that the number of warp yarns in the top
layers of warp yarns is greater than in the bottom layers so as to
provide sufficient lengths for the attachment tabs.
The weaving may be of the interlock type, as shown. Other
three-dimensional or multilayer weaves may be used, e.g. such as
multi-plain or multi-satin weaves. Reference may be made to
document WO 2006/136755.
After weaving, the blank 100 may be shaped in order to obtain a
ring sector reform 110 without cutting any yarns, as shown
diagrammatically in FIG. 6, where there can be seen only the warp
yarns and the envelope outline of the preform 110.
FIG. 7 shows successive steps of one way of making a CMC ring
sector, e.g. with a fiber preform made of SiC fibers.
In step 70, a continuous fiber strip is woven with SiC fiber yarns,
the strip having its longitudinal direction in the warp direction,
in the manner shown in FIGS. 5A and 5B.
In step 71, the fiber strip is treated to eliminate the sizing
present on the fibers and also the presence of oxide at the surface
of the fibers. The oxide is eliminated by acid treatment, in
particular by immersion in a bath of hydrofluoric acid. If the
sizing cannot be eliminated by the acid treatment, prior treatment
for eliminating the sizing is performed, e.g. by decomposing the
sizing by short heat treatment.
In step 72, a thin layer of interphase coating is formed on the
fibers of the fiber strip by chemical vapor infiltration (CVI). By
way of example, the material of the interphase coating is pyrolytic
carbon or pyrocarbon (PyC), boron nitride (BN), or a boron-doped
carbon (BC, e.g. having 5 atomic percent (at %) to 20 at % of B,
the balance being C). The thin layer of interphase coating is
preferably of small thickness, e.g. no greater than 100 nanometers
(nm), or indeed no greater than 50 nm, so as to conserve good
capacity for deformation in the fiber blanks. The thickness is
preferably not less than 10 nm.
In step 73, the fiber strip together with its fibers coated in a
thin layer of interphase coating is impregnated with a
consolidation composition, typically a resin that is optionally
diluted in a solvent. It is possible to use a carbon-precursor
resin, e.g. a phenolic or a furanic resin, or a ceramic-precursor
resin, e.g. a polysilazane or a polysiloxane resin that is a
precursor of SiC.
After drying by eliminating any solvent from the resin (step 74),
individual fiber blanks 100 are cut apart (step 75).
In step 76, a fiber blank as cut out in this way is shaped and
placed in a mold, or shaper, e.g. made of graphite, for shaping so
as to obtain a preform 110 of a shape that is close to the shape of
a ring sector 10 that is to be fabricated.
Thereafter, the resin is cured (step 77) and the cured resin is
pyrolyzed (step 78). Curing and pyrolysis may be performed one
after the other by progressively raising the temperature in the
mold.
After pyrolysis, a fiber preform is obtained that has been
consolidated by the pyrolysis residue. The quantity of
consolidation resin is selected so that the pyrolysis resin bonds
together the fibers of the preform sufficiently to enable the
preform to be handled while conserving its shape without the
assistance of tooling, it being understood that the quantity of
consolidation resin is preferably selected to be as small as
possible.
A second interphase layer may be formed by CVI (step 79) if needed
in order to obtain overall a fiber-matrix interphase of thickness
that is sufficient to perform an embrittlement relief function for
the composite material. The second interphase layer may be a
material selected from PyC, BN, BC, and need not necessarily be the
same as the material of the first interphase layer. As is known,
such interphase materials are capable of performing a function of
relaxing stresses at the bottoms of cracks that reach the
interphase through the matrix of the composite material, thereby
avoiding or slowing down propagation of cracks through the fibers,
which would otherwise cause the fibers to rupture, thus making the
composite material less fragile. The thickness of the second
interphase layer is preferably not less than 100 nm.
It is preferred to form the interphase from two interphase layers,
as described above. The first interphase layer contributes to
avoiding excessive adhesion on the fibers of the residue of
pyrolyzing the consolidation resin.
Thereafter the consolidated preform is densified with a ceramic
matrix. The densification may be performed by CVI, with it then
being possible for the formation of the second interphase layer and
for the densification with the ceramic matrix to follow on one from
another in the same oven.
Using CVI to densify a preform with a ceramic, in particular an SiC
matrix, is well-known. A reaction gas containing methyl
trichlorosilane (MTS) and gaseous hydrogen (H.sub.2) may be used.
The consolidated preform is placed in the enclosure, without using
tooling to keep it in shape, and the gas is introduced into the
enclosure. Under controlled conditions, in particular of
temperature and pressure, the gas diffuses through the pores of the
preform in order to deposit the SiC matrix by means of a reaction
between the constituents of the gas.
CVI densification of the consolidated preform may be performed
using a matrix other than SiC, in particular using a self-healing
matrix, with examples of self-healing matrix phases being a ternary
Si--B--C system or boron carbide B.sub.4C. Reference may be made to
documents U.S. Pat. No. 5,246,736 and U.S. Pat. No. 5,965,266 that
describe obtaining such self-healing matrices by CVI.
The densification may be performed in two successive steps (steps
80 and 82) that are separated by a step 81 of machining the part
for fabrication to the desired dimensions. The second densification
step serves not only to finish off densifying the composite
material to the core, but also to form a surface coating on any
fibers that might have been laid bare during machining.
It should be observed that pre-machining, or trimming, may be
employed between steps 77 and 78, i.e. after curing and before
pyrolyzing the resin.
After densification, the layer of abradable coating may be formed,
e.g. by physical gas deposition, in known manner.
With reference to FIG. 7, the use of SiC fiber yarns for forming
the fiber reinforcement of the composite material is mentioned.
Naturally, it is possible to use fibers made of some other ceramic
or carbon fibers. When using carbon fibers, step 71 is omitted.
In the description above, ring sectors are made having connection
tabs that present a meridian section that is S-shaped.
In a variant, it is possible for the connection tabs to have a
meridian section that is L-shaped, like the tabs 14', 16' of the
ring sector 10' shown in FIG. 8.
* * * * *